illusion

nounil·lu·sion \il-ˈü-zhən\(Medical Dictionary)

Medical Definition of ILLUSION

1

: a misleading image presented as a visual stimulus

2

a: perception of something objectively existing in such a way as to cause misinterpretation of its actual nature; especially:optical illusion—compare delusion 2 b:hallucination 1 c: a pattern capable of reversible perspective